The present invention relates to a memory storage device and, more specifically, to the use of a ferrimagnetic layer and an anti-parallel layer in a free layer of a magnetic tunnel junction (MTJ).
Magnetic Random Access Memory (MRAM) technology utilizes storage cells, for example, MTJs, which generally each have at least two magnetic regions or layers with an electrically insulating barrier layer between them. The data storage mechanism relies on the relative orientation of the magnetization of the two layers, and on the ability to discern this orientation through electrodes attached to these layers. For background, reference is made to U.S. Pat. Nos. 5,650,958 and 5,640,343 issued to Gallagher et al. on Jul. 22, 1997 and Jun. 17, 1997, respectively, which are incorporated herein by reference.
An MTJ is a device that typically includes two ferromagnetic electrodes separated by a thin insulating layer. MTJs are based on the phenomenon of spin-polarized electron tunneling. The insulating layer is thin enough that tunneling occurs between the ferromagnetic electrodes.
A conventional MTJ device contains a xe2x80x9cfreexe2x80x9d ferromagnetic layer, e.g. cobalt (Co), and a xe2x80x9cpinnedxe2x80x9d ferromagnetic layer, e.g. cobalt-iron (Coxe2x80x94Fe), separated by the insulating tunneling layer, e.g., aluminum oxide. The xe2x80x9cpinnedxe2x80x9d ferromagnetic layer has a magnetization that is oriented in the plane of the layer but is fixed so as to not be able to rotate in the presence of an applied magnetic field in the range of interest. The pinned ferromagnetic layer is fixed by interfacial exchange coupling with an adjacent antiferromagnetic layer. The xe2x80x9cfreexe2x80x9d ferromagnetic layer has a magnetization that is able to be rotated in the plane of the layer relative to the fixed magnetization of the pinned ferromagnetic layer.
The tunneling phenomenon is electron spin dependent, making the magnetic response of the junction a function of the relative orientations and spin polarizations of the two electrodes. The amount of tunneling current that flows perpendicularly through the two ferromagnetic layers and the intermediate tunnel barrier depends on the relative magnetization directions of the two ferromagnetic layers. If the magnetic axes of the two ferromagnetic layers are aligned with each other, electrical resistance in the MTJ device is lower resulting in a higher level of current through the MTJ device. If the magnetic axes of the ferromagnetic layers are opposite one another, a higher level of resistance will result causing a lower current through the MTJ device. Thus, the magnetic state of the free layer can be read by the measured level of current through the MTJ device.
In operation as a memory device, an MRAM device can be read by measuring the tunneling resistance to infer the state of the magnetization of a free or storage layer with respect to a pinned layer in an MTJ. An MRAM device can be written by reversing the free layer magnetization using external magnetic fields. If the free layer is imagined as a simple elemental magnet which is free to rotate but with a strong energetic preference for aligning parallel or antiparallel to the x axis, and if the pinned layer is a similar elemental magnet but frozen in the +x direction, then there are only two relative magnetization states possible for the free and pinned layers of the device, aligned and anti-aligned.
Various parameters are of interest in evaluating the performance of these devices. First, the variation in resistance between the two storage states is described by the magnetoresistance MR, i.e., the percentage change in resistance between the two states. Historically, ferromagnetic materials with a higher saturation magnetization Ms were used to obtain junctions with higher MR values (see, for example, R. Meservey and P. M. Tedrow, Phys. Rep. 238, 173 (1994)). More recently, it has been shown that there is only a weak link between the magnitude of the saturation magnetization and MR of MTJs containing electrodes formed from alloys of Co, Fe and Ni (D. J. Monsma and S. S. P. Parkin, Appl. Phys. Lett. 77, 720 (2000)).
Second, the coercive field is of interest since fields generated by currents along wires in an array of storage cells will need to be able to rotate the magnetization of the storage layer. As the capacity of memory arrays increases, the MTJ area will inevitably become smaller and more dense. As this happens, the switching field (also termed the coercive field Hc) rises roughly inversely with lateral dimension, for the same material and thickness and the same aspect ratio and shape of the device. Using current designs, one quickly reaches a situation where junction size, dictated by needs for higher, density causes the coercive fields to become unmanageably large.
In addition to these issues, there are other problems that arise when trying to push the size of the devices down into the sub-micron regime. First, there are strong demagnetizing fields which will tend to cause the bit to xe2x80x9cerasexe2x80x9d over time. Second, the demagnetizing field is non-uniform. In particular, the demagnetizing field is strongest close to the edges of the devices. Thus, control over the uniformity of the device is most important at the very spot where there is the most difficulty in fabrication. Small defects at the edge of the magnetic element can therefore lead to nucleation or pinning sites for unwanted micromagnetic structures, resulting in unpredictability of the junction properties. Third, using polycrystalline material such as permalloy can lead to increased variation in device properties because of random orientation of the microcrystallites. In very small devices, the statistical fluctuations due to the grain structure will become much more pronounced. These grains also can cause variations in the tunneling properties between electrodes in the MTJ, causing further uncertainty or variation in device properties.
Several solutions have been proposed in order to alleviate this rising coercivity problem. First, one may reduce the saturation magnetization of the storage electrode, since Hc scales with Ms. However, many low Ms materials, for example, formed by alloying Co or Fe with non-magnetic elements, give low MR. Second, one may reduce the thickness of the magnetic electrode, since Hc scales with electrode thickness. Current junctions, however, are on the edge of continuity due to their extremely thin electrodes and further reduction is difficult at best.
Therefore, a need exists for a new approach to fabricating MTJ devices, while still leaving freedom to tune the coercive field and provide an appreciable MR.
In a first aspect of the invention, an MTJ device is provided. The MTJ device includes a free layer and a pinned layer separated by a barrier layer. The free layer of the MTJ includes a ferrimagnetic material and an anti-parallel layer having a magnetic moment that is substantially anti-parallel to a magnetic moment of the ferrimagnetic layer at least within a predetermined temperature range of the magnetic tunnel junction device. The ferrimagnetic layer and the anti-parallel layer may be separated by a spacer layer, or may be directly adjacent to each other.
In a preferred embodiment, the anti-parallel layer includes a ferromagnetic material. However, the anti-parallel layer can also include a ferrimagnetic material having different magnetic properties than the ferrimagnetic material within the free layer. For example, the ferrimagnetic material used in the anti-parallel layer can have a different compensation temperature than the ferrimagnetic material of the free layer.
In another aspect of the invention, a memory array is provided. The memory array includes a plurality of memory cells, at least one of the memory cells including an MTJ having a free layer that includes a ferrimagnetic material and an anti-parallel layer having a magnetic moment substantially anti-parallel to a magnetic moment of the ferrimagnetic layer at least within a predetermined temperature range of the magnetic tunnel junction device.